1. Field of the Invention
The present invention relates to a method of manufacturing a wafer, in which a heterogeneous material compound is detached at a pre-determined detachment area of the compound, and the compound is subject to a thermal treatment.
2. Background of the Invention
Heterogeneous material compounds or heterostructures having a thin layer attached to a receiver substrate with a different thermal expansion coefficient have attained considerable industry attention in the fields of microelectronics, optoelectronics and micromechanics. Such structures can be fabricated using deposition processes based on electroplating, evaporation, spinning, etc. In other approaches, a bulk material is bonded to a receiver substrate and thereafter thinned down, either by a chemical mechanical polishing or by etching of the material. These methods mostly have a very low efficiency due to a lot of process steps and the resulting structures often cannot be produced with the required quality.
U.S. Pat. No. 5,877,070 discloses a method of manufacturing a wafer of the above-mentioned type. This method uses a modified variant of the well known SMARTCUT® process to transfer a thin film onto a hetero-substrate. Characteristic steps of this process are schematically shown in FIGS. 18a to 18c. 
As shown in FIG. 18a, a donor wafer 91 and a receiver wafer 92 of materials with different thermal expansion coefficients are provided. With reference to FIG. 18b, the donor wafer 91 is implanted through its surface 93 with ions 96, creating a pre-determined zone of weakness 95 at or in the vicinity of a certain implantation depth d of the donor wafer 91.
Then, the implanted donor wafer 91 is annealed using a tempering device 90, as shown in FIG. 18c. This annealing step directly after the implanting step results in a weakening of the pre-determined detachment area 95 due to formation and growth of micro-cracks in the implanted region. The temperature used at that annealing step must be adjusted at a relatively low value to prevent the formation of surface blisters induced by ion implantation, which would prevent subsequent bonding of the donor substrate with a second substrate. Therefore, the weakening effect resulting from this annealing step is relatively minor.
As shown in FIG. 18d, the implanted and annealed donor wafer 91 is bonded with the receiver wafer 92 at the implanted surface 93 of the donor wafer 91 resulting in a heterogeneous wafer compound 99. This is followed by a second thermal treatment in a tempering device 90′, as shown in FIG. 18e. The second thermal treatment causes further growth, overlapping and coalescence of the micro-cracks induced by ion-implantation, which detaches the wafer compound 99 at the pre-determined detachment area 95 when an energy corresponding to a budget of thermal detachment is reached for the respective compound.
The budget of thermal detachment is a certain thermal budget corresponding to the limit for thermal detachment or cleaving of a material, which is 100% of the necessary energy at which detachment occurs thermally. The used temperature-time-dependency of the budget of thermal detachment follows the Arrhenius Law in which the reciprocal of the annealing time is proportional to the exponent of the reciprocal of the annealing temperature. The budget of thermal detachment of heterogeneous bonded structures is dependent on a number of material, environmental and technological parameters like the kind of material, implantation conditions and bonding conditions.
The above described second thermal treatment must be carried out at relatively low temperature at which the bonded wafer pair suffers from degradation due to the different thermal expansion coefficients of the materials of the wafers 91, 92. This leads to an expanded annealing time for detachment of the wafer compound 99 to transfer a thin layer 97 of the donor wafer 91 to the receiver wafer 92.
In a further approach, U.S. Pat. No. 5,877,070 suggests an additional implantation step using a boron implantation to lower the detachment temperature. This method results in disadvantageous boron doping of the surrounding layers and is, especially due to the additional implantation step, expensive and time-consuming.
Another attempt, which has been presented for instance by Aspar et al. in the Proceedings of MRS, 1998, uses a high dose hydrogen implantation to facilitate detachment of a wafer of a heterogeneous wafer compound at the implanted area in an annealing step. However, this high dose ion implantation raises the cost of manufacture.
FR patent application 2,755,537 A describes a method to transfer a thin layer to a heterostructure in which an implanted substrate is thinned down after bonding of this substrate with another substrate to limit a sudden stress variation during thermal detachment causing indefinite breakage of the compound. This method causes an additional process step and results in a significant material consumption because the removed material is lost.
FR patent application 2,748,851 A discloses a method to detach a structure at lower temperatures using a combination of a heat treatment and mechanical efforts such as traction, shearing or bending forces. Such forces can be applied with a tool, a fluid or with another source of mechanical energy, for instance, as described in European patent application 0 977 242 A2, with a jet.
PCT publication WO 01/80308 A2 proposes an injection of energy pulses, such as laser pulses, into a structure with an embrittled zone to transfer a thin layer of silicon onto a SiO2 substrate.
Thus, there is a need for manufacturing a wafer with an easy and effective detachment of a heterogeneous material compound at a reduced risk of an undefined breaking of the compound, and this is now provided by the present invention.